The present invention relates to a paper machine for manufacturing a web of soft crepe paper.
It is known to utilize through drying in the manufacture of soft crepe paper in order to pre-dry the web without compressing it. Through drying is effected using a through drying machine based on two different techniques, said through drying machine having a cylinder with perforated shell around which the web runs accompanied by a perforated belt of wire type. According to one technique air is pressed and/or sucked from outside and into the through-blow cylinder. In the other technique air is pressed and/or sucked in the other direction, i.e. from the inside and out through the through-blow cylinder.
Solutions have been suggested for the "from the inside and out" technique, see U.S. Pat. No. 3,303,576, U.S. Pat. No. 4,036,684 and U.S. Pat. No. 5,274,930 (FIG. 3B), for example, wherein after its removal from the forming unit and up to the drying cylinder or a transfer roll beside it, the wire-supported web is not subjected to any compression by guide rolls. However, the wire running outside the web in the through drying machine may have a certain compressive action on the web. Due to the pressure drop that occurs through the web and the wire, the web is lifted from the through-blow cylinder so that the normal pressure between this and the web becomes extremely low. However, lifting the web from the through-blow cylinder gives rise to loss of drying air since, instead of passing straight through the web, the drying air flows out to the sides through the gap formed when the web is lifted from the through-blow cylinder. Besides these losses of drying air, the method results in uneven drying of the web seen in cross section. In order to reduce the losses of drying air disappearing from the side edges of the web, the velocity of the air which is to pass through the web must be limited. The through drying machine thus has limited drying effect, and also limited usefulness as regards the type of pulp formed to a web with sufficient permeability to air for the through drying machine with reduced effect.
Solutions for the "from the outside and in" technique have been proposed, see U.S. Pat. No. 3,812,000 and U.S. Pat. No. 3,821,068, for example, in which the web runs through the through drying machine with one surface being exposed to the drying air. The web carried by the wire reaches the drying cylinder via guide rolls, the surface of the web exposed to drying air being in contact with the guide rolls, while at the same time the wire exerts a pressure on the web so that it is compressed in an undesirable manner. Since the web is in direct contact with said guide rolls, there is a risk of fibres adhering to them, thereby causing fibre losses and the web being damaged by accumulations of fibres that have gradually collected on the guide rolls. The guide rolls must therefore be cleaned. An arrangement with guide rolls entails increased costs and requires space, which increases the total space requirement for the paper machine. U.S. Pat. No. 5,274,930 (FIGS. 2 and 3A) also proposes drying air being blown through from the outside to the inside, however, it must first pass through the wire covering the web which results in considerably poorer drying effect due to restricted air temperature in comparison with a web having one side exposed to the drying air as described in U.S. Pat. No. 3,812,000, where every small surface unit is in prolonged contact with drying air as the web passes through the through drying machine. Furthermore, the wire carrying the web in U.S. Pat. No. 5,274,930 must be passed around guide rolls up to the drying cylinders. As mentioned earlier, an arrangement with guide rolls involves increased costs and requires space, thereby increasing the total space requirement of the paper machine. Although the web is not compressed in this case, when it passes over the guide rolls on the outside of the wire, it is still subjected to compression when it passes through the through drying machine due to the pressure exerted by the wire on the web during its movement around the through-blow cylinder and this pressure increases since the drying air acts directly on the wire.